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(CNN)US President Donald Trump may be about to declare that Iran is no longer in compliance with the international nuclear agreement, but the country is on a roll across the Middle East. Iran has great influence in Iraq and Syria, strong relations with Russia and Turkey and its arch-rival, Saudi Arabia, is undergoing a difficult generational transition.

Iran had dug deep into its pockets to support the regime of Bashar al-Assad in Syria long before sanctions were eased as part of the 2015 nuclear deal. Militias trained and armed by Iran's Quds force have played a pivotal role in the fighting against rebel factions, especially around Aleppo. Weapons and cash have also flowed from Tehran to Damascus.

In Iraq, pro-Iranian militias such as the Badr Brigade came to the rescue when the Iraqi army collapsed in 2014 and they led the fight against ISIS in places like Tikrit and Tal Afar. They are now "partners" of Iraq's defense forces but not truly controlled by Baghdad.

These militias -- the Popular Mobilization Units -- have even turned up on Iraq's border with Syria, bringing a step closer Iran's dream of linking Tehran, Baghdad, Damascus and Beirut in a Shia arc of influence. With elections in Iraq next year, the militias are looking toward a political role.

History is a powerful driver here. Iran is scarred by the horror of its eight-year war against Saddam Hussein in the 1980s and it has always wanted a pliable neighbor. As a former member of Iraq's Parliament, Sami al-Askari, put it: "The American approach is to leave Iraq to the Iraqis. The Iranians don't say leave Iraq to the Iraqis. They say leave Iraq to us."

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After the nuclear deal was signed, Barack Obama said he hoped it would encourage Iran to "behave differently in the region, to be less aggressive, less hostile."

Critics said that would never happen. And in the past two years, Iran has developed its ballistic missile program, taken Qatar's side in its squabble with Saudi Arabia and, according to the Saudis, armed the Kingdom's Shia opponents in Yemen.

'Wrong choices'

Iranian Foreign Minister Javad Zarif argues that missteps by the country's neighbors and their western allies are behind widening Iranian influence in the Middle East.

Iran doesn't deny that its influence has spread. Foreign Minister Javad Zarif says it's because of the "actions, mistakes, and wrong choices" of Iran's neighbors and their western allies.

"It was not Iran that prevented a churlish Saudi Arabia from opening an embassy in Baghdad for a decade after the fall of Saddam, nor was it Iran that insisted on war with Yemen or an embargo of Qatar," he wrote in the Atlantic this week.

Since its neighbors closed their borders in June, Qatar has turned to Turkey and Iran to import essential supplies. Iranian shippers have begun services across the Gulf to the Qatari capital of Doha.

From Yemen to Lebanon and Iraq, Iran has adroitly exploited conflict and weak states to its benefit. It's not an expensive effort (at least set against Saudi Arabia's multibillion-dollar arms buys), relying on volunteers ready for martyrdom and an apparently inexhaustible supply of armored trucks and light weapons.

It's difficult to know how far relief from sanctions has fueled Iran's ability and desire to project its power across the region. US officials estimate that Iran recovered some $100 billion in frozen assets. But it also has a lot of long-standing debts, and a vast infrastructure program at home to pay for.

A distorted economy

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But although Iran may seem newly confident abroad, at home it faces myriad problems. Youth unemployment is high, corruption pervasive and the economy distorted by inefficient state-run companies, many of them owned by the Islamic Revolutionary Guard Corps (IRGC).

Over the years, the IRGC has gobbled up companies in construction, banking, insurance and telecommunications. President Hassan Rouhani has publicly complained about the Guards' outsize role in the economy, and the accompanying graft.

The uncertain business environment has deterred international investors, even though Iran ought to be an enticing market, with its large, young urban population. Iran sits 120th on the World Bank's Ease of Doing Business index.

There are other limits to Iranian expansionism. It speaks of a strategic alliance with Moscow, but Russian (power) and Iranian (ideological) aims in Syria are not always aligned. And in the absence of consistent US engagement, the Russians have stepped up as the go-to power in the Middle East, balancing their relationships. Last week, the Saudi King was in Moscow buying Russia's most advanced anti-missile system.

Similarly, Israeli Prime Minister Benjamin Netanyahu has repeatedly warned Russian President Vladimir Putin that Israel won't tolerate Iranian dominance in Syria. Netanyahu told CNN in September that Iran was trying to "colonize'' Syria with the aim of "destroying us and conquering the Middle East."

In Israel's eyes, that aim is served by Iran's support for the Lebanese Shia militia Hezbollah and Hamas in Gaza, along with its ongoing ballistic missile program.

Both Saudi Arabia and Israel will be delighted if Trump declares that Iran is no longer in compliance with the nuclear deal and Congress adopts new sanctions. Ever since the Islamic Revolution in 1979, Sunni-dominated Saudi Arabia has seen the Shia fundamentalism across the Persian Gulf as an existential threat. Today, it accuses Iran of meddling in Qatar, Bahrain and Yemen, and even attempts to incite Saudi Arabia's own Shia minority.

But Iran feels it has a historical purpose. Its constitution charges the military to "fulfill the ideological mission of jihad in God's way, extending the sovereignty of God's law throughout the world." It's a mission with an uncompromising Shia identity, and one that has brought Iran to the shores of the Mediterranean.